Nature and development of plants . s canalways l»- detected in the grain of wood because the) run a<or at righl angles to the xylem cells < Fig. 57. mr). \& a rule NATURE OF PLANTS 81 the wood cells of the stem remain alive for only a few fact is usually apparent in any log or stump where it willbe seen that the outer annular rings are of a lighter color, thesap wood, and the older parts are of a darker color, the heartwood. The sap wood is active in the transport of water andcontains many living cells. For this reason it is not so valuablefor lumber. The heart wood is compose
Nature and development of plants . s canalways l»- detected in the grain of wood because the) run a<or at righl angles to the xylem cells < Fig. 57. mr). \& a rule NATURE OF PLANTS 81 the wood cells of the stem remain alive for only a few fact is usually apparent in any log or stump where it willbe seen that the outer annular rings are of a lighter color, thesap wood, and the older parts are of a darker color, the heartwood. The sap wood is active in the transport of water andcontains many living cells. For this reason it is not so valuablefor lumber. The heart wood is composed of dead cells andwhile the most serviceable for building purposes, it is of littleuse to the tree as we often see a vigorous tree whose heart woodhas been largely destroyed by decay. 40. Other Types of Stems.—The cone bearing trees, such asthe pines and spruces, have essentially the same arrangementof tissues and mode of growth as noted above in the xylem, however, consists entirely of tracheids (Fig. 58). Fig. 60. Cross-section of a single vascular bundle of corn stem: ; x. small cells of the xylem; v. vessels of xylem; st, stereome thatforms a sheath about the bundle; p, parenchyma of the stem. 7 82 ED >NGATU >\ < IF THE STEM with the exception of a few small spiral vessels that are formedas the first cells of the vascular bundles. The monocotyledons, plants distinguished usually by theirparallel veined leaves and single seed leaf, like the palms, lilies,ses, etc., are characterized by stems that do not increasematerially in diameter. Growth is largely confined to the topof the stem and consequently it can only elongate, forming avery regular, columnar trunk. The reason of this is apparentwhen cross sections of such stems are examined I Fig. 45. D).The vascular bundles are more or less scattered throughout thestem and it is especially noteworthy that there is no cambium separating the xylem and phloem. It will he seen by examining Fig.
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